This invention pertains to a synchronous control system for a pair of hydrostatic transmissions for sensing a variation in the output speeds of the two transmissions and making a correction in the speed of one of said transmissions to synchronize the speeds thereof. More particularly, the synchronous control system is a tracking control in a vehicle drive, having the pair of transmissions, each driving a propulsion member, whereby a difference in speed of the propulsion members is sensed and a correction is made in the speed of one hydrostatic transmission and associated propulsion member to bring the speeds back to synchronization, to provide for straight line movement of the vehicle. Vehicles using a dual path hydrostatic transmission system frequently exhibit a tendency to mistrack when the operator intends for the vehicle to go in a straight line. This mistracking occurs because of manufacturing tolerances and inefficiences of the transmissions. U.S. Pat. No. 3,247,919, owned by the assignee of this application, discloses an early form of steering control, wherein a differential sensed the output speed of two transmissions and controlled a pump to provide a signal to a steering control cylinder in order to correct any mistracking.
The use of two pumps, driven one by each of two hydrostatic transmissions to obtain tracking control, is shown in Chatterjea U.S. Pat. No. 3,677,362. A similar type structure is shown in Ward U.S. Pat. No. 3,795,107 and the latter patent makes reference to several earlier patents as disclosing tracking controls.
Ward U.S. Pat. No. 3,862,668 discloses a hydrostatic transmission control system wherein a rotary valve has a pair of rotatable valve elements driven, one by each of a pair of hydrostatic transmissions, and with the rotary valve functioning to deliver a correctional control signal to a displacement control for one of the hydrostatic transmissions. A modified form of steering control valve is disclosed in this patent which acts to block the straight tracking valve assembly when a steering control valve is actuated. This may be for the purpose of avoiding a possible tendency for the transmissions to gradually reduce to zero speed when a steering control valve is actuated in the primary embodiment.
A synchronous control for cylinder motors associated with a hydraulic metal working press and using a rotary valve having a pair of relatively rotatable valve members controlling communication of a pair of lines to tank is shown in Zimmerman U.S. Pat. No. 2,859,591.